Disposable diapers generally comprise a liquid absorbent layer sandwiched between an external, liquid impervious layer, and an internal, liquid permeable liner. In order to perform its function well, the absorbent layer has to be capable of absorbing large quantities of liquids, many folds larger than its own weight. The main component of the absorbent layer is a fibrous substance such as wood pulp fibers, and in order to improve the liquid absorbance, this layer is supplemented with granules of super absorbent polymers (SAP) which typically constitute about 17-20% of the absorbent layer. SAP is capable of absorbing about 35-40 times its weight in liquid whereas wood pulp fibers are capable of absorbing only about 7 times their weight in liquid. However, SAP, is an expensive material and costs about three times more than wood pulp fibers.
Good liquid absorbance is most important in the front half of the diaper at the area coming in contact with the child's genitals and crotch region. Thus, in view of its high price tag it would have been desirable to have the bulk of the SAP in a diaper in the front half and only a small part of the SAP in the rear half of the diaper. A very desirable ratio between the amount of SAP in the front and in the rear half of the diaper would be about 8:2, i.e. 80% of the SAP in the front half of the diaper and only 20% in the rear half. However, none of the methods available to date in preparing a disposal diaper allows such a distribution of SAP.
The absorbent material of disposable diapers is typically formed as a continuous sheet, which is then sandwiched between the liquid impervious layer on the one hand and the liquid permeable liner on the other hand and cut according to the desired shape. The absorbent sheet is typically produced in a vacuum chamber having a moving screen at its bottom through which the vacuum is applied. Fibers are introduced into this chamber by a stream of air and are then deposited on the screen. The moving screen is typically a foraminous belt on which a continuous sheet of absorbent material is formed; or a rotating drum having shaped depression in which the fibres are deposited, whereby the formed sheet assumes some of the shape characteristics of the subsequently formed disposable diapers. SAP is discharged from a pipe situated above the belt in the chamber and thus applies the SAP into the forming blend of material which forms the sheet. In accordance with known means the SAP was either continuously applied, in which case it became evenly distributed along the formed sheet, and eventually evenly distributed in the produced disposable diapers, or alternatively, the discharge pipe was provided with an intricate and very expensive valve mechanism which allowed the controlled intermittent discharge of SAP only on portions which eventually formed the front half of the disposable diaper.
None of these SAP application means enables to obtain about an 8:2 distribution ratio mentioned above.
It is an object of the present invention to provide novel means for the preparation of an absorbent sheet comprising a blend of two substances, one being a fibrous substance and the other being a particulate substance, in which sheet, the second substance is unevenly distributed.
A particular object of the present invention is to provide means for the preparation of an absorbent sheet for use in disposable diapers in which SAP is located predominantly in the front half.
Various other objects of the invention will be eleviated in the following description and claims.